This invention relates generally to coherent, surface emitting, semiconductor laser arrays. More particularly, the invention relates to the fabrication of two-dimensional phase-locked, surface emitting semiconductor laser arrays in a monolithic wafer.
The potential for monolithic integration of two-dimensional laser arrays has stimulated the investigation of various means of forming semiconductor lasers that emit laser energy normal to the plane of the wafer. Two general types of surface emitting lasers are known to have been constructed. One has a short oscillator cavity perpendicular to the plane of the wafer and the other has its oscillator cavity in the plane of the wafer with emission normal to the surface being accomplished by a turning mirror or a distributed Bragg reflector in the surface of the wafer. Further, investigation of means for forming diode lasers that are simultaneously surface emitting and two dimensionally coupled has been stimulated by the potential of coupling several monolithically formed diode lasers together to enable tuning all to a single wavelength, to promote phase-locking for spatially coherent emission and to provide high power outputs. A structure to accomplish two-dimensional coupling and surface emission is proposed by the present invention.
Turning mirrors have been successfully fabricated in the surface of semiconductor materials, e.g., GaAs, AlGaAs, by chemical etching, mass transport, ion-milling, and ion-beam assisted etching. Focused ion beam (FIB) micromachining of integrated optical structures recently has been reported. The FIB micromachining of high-quality optical surfaces to create low threshold diode lasers, coupled-cavity lasers and surface emitting diode lasers is described in our copending application Ser. No. 858,357, entitled "Focused Ion Beam Micromachining of Optical Surfaces in Materials", filed May 1, 1986, and now U.S. Pat. No. 4,698,129, and in our published reports entitled "Focused-Ion Beam Micromachined AlGaAs Semiconductor Laser Mirrors," Electronics Letters, Vol. 22, No. 13, pp. 700-702 (1986); "CW Operation of Widely and Continuously Tunable Micromachined-Coupled-Cavity Diode Lasers," Electronics Letters, Vol. 22, No. 17, pp. 919-921 (1986); and "300 mW Operation of a Surface Emitting Phase-Locked Array of Diode Lasers," Electronics Letters, Vol. 23, No. 3, pp. 130-131 (1987), all of which are incorporated herein by this reference.
Optical-quality FIB micromachining makes possible the preparation of intricately curved semiconductor laser surfaces, such as parabolic turning mirrors, as well as other structures required for wafer scale monolithic integration. One such structure, found to have particularly useful surface emitting and injection-coupling features, is proposed by the present invention.
Diode lasers are typically manufactured from a wafer consisting of, for example, a GaAs substrate on which layers of AlGaAs and GaAs have been grown epitaxially to form a p-n junction and an optical waveguide. The waveguide serves to confine the radiation emitted by recombining electrons and holes at the p-n junction to a thin layer in the plane of the wafer. Long, narrow laser cavities, e.g., 1.times.4.times.300 .mu.m, are formed by restricting the light laterally by either a lateral waveguide, in the case of index-guided lasers, or by confining the current carriers laterally, in the case of gain-guided lasers. The laser oscillator mirrors which define the longitudinal dimension of a diode laser are conventionally formed by cleaving the wafer crystal perpendicular to the laterally confining feature, be it an optical waveguide or a high-current, high-gain stripe.
By micromachining a vertical output mirror and a 45.degree. planar turning mirror perpendicular to a high-gain stripe, the laser energy which would otherwise be emitted in the plane of the substrate and in the direction of the stripe (longitudinal), may be reflected in a direction generally normal to the surface of the wafer.
Planar mirrors are not an ideal geometry for reflecting laser energy normal to the surface. This is because the light wave emerging from the vertical emitting region within the monolithic structure is not planar, but rather is strongly divergent. A parabolic, mirrored surface located at twice the parabola's focal length from an emitting region will ideally redirect all incident laser energy normally away from the surface in a diffraction-limited beam. If channels having symmetric, thus-located parabolic sections are micromachined into the surface of the diode laser wafer perpendicular to the stripe thereof, then laser energy from adjacent emitting regions incident upon either parabolic surface will be reflected normal to the surface of the array. By injecting laser energy from an emitting region on one side of the channel to the emitting region on the other side of the channel, the stripes that are interrupted by the channels may be longitudinally injection coupled to one another.
It is desirable to provide a surface emitting semiconductor laser that takes advantage of the well-known lateral evanescent coupling of multiple stripe arrays and that also takes advantage of the longitudinal, injection-coupled laser diode pairs described immediately above. The combination of these two coupling structures makes possible the realization of two-dimensional, phase-locked, surface emitting, semiconductor laser arrays of greatly increased coherent output power.
Accordingly, it is an object of this invention to provide a coherent, phase-locked, two-dimensional, surface emitting semiconductor laser array in a monolithic substrate.
Another object is to provide such an array that is compatible with known micromachining processes.
Yet another object of the present invention is to provide a laser array that makes use of conventional diode laser materials.
In the preferred embodiment of the present invention, FIB micromachining enables the preparation of such channels in the surface of multiple stripe diode laser wafer material to produce a two-dimensionally coherent, surface emitting semiconductor laser array in a monolithic structure having a plurality of longitudinally injection-coupled and laterally evanescently coupled lasers therein. The surface emitting and injection coupling geometries are well-defined and highly controlled by the FIB micromachining process, and enable the production of high-power, low divergence outputs. The injection coupling of adjacent emitting regions is accomplished by insubstantially interrupting the reflective, parabolic surfaces with dual channel-lengthwise, semi-transmissive regions located opposite dual emitting regions. These opposing, injection coupling surfaces act as conduits for laser energy to pass through, thus longitudinally coupling adjacent laser columns, while insubstantially interrupting the reflective, parabolic surfaces that give rise to surface-normal emission.
These and other objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following description of the preferred embodiment.